<!DOCTYPE html>
<script src="https://how2j.cn/study/js/jquery/2.0.0/jquery.min.js"></script>
<link href="https://how2j.cn/study/css/bootstrap/3.0.3/bootstrap.min.css" rel="stylesheet">
<script src="https://how2j.cn/study/js/bootstrap/3.3.6/bootstrap.min.js"></script>
  
<div class="btn-group">
   <button type="button" class="btn btn-default">默认</button>
   <button type="button" class="btn btn-default dropdown-toggle"
      data-toggle="dropdown">
      <span class="caret"></span>
 
   </button>
   <ul class="dropdown-menu" role="menu">
     <li>
         <a href="#">HTML</a>            
      </li>
      <li role="presentation">
         <a href="#">CSS</a>             
      </li>
  
      <li role="presentation">
         <a href="#">Javascript</a>          
      </li>
      <li role="presentation">
         <a href="#">AJAX</a>            
      </li>
   </ul>
</div>
 
<div style="height:100px"></div>